The present invention relates to a self-oscillating circuit for driving a gas discharge lamp, particularly for use in motor vehicle headlights or spotlights.
More specifically, the invention relates to a self-oscillating circuit comprising
an inverter having two circuit branches connectable to a DC voltage source and including respective electronic switching means in series with a respective primary winding; the primary windings of the circuit branches being inductively coupled to the same secondary winding connected to the lamp;
an impedance in which variable as a function of frequency, connected in series with the lamp; and
control circuit means operable to render the electronic switch means alternately conductive in such a way that an alternating voltage develops across the terminals of the said secondary winding, the control circuit means having a control input intended to receive a control signal to cause a variation in the frequency of the alternating voltage in such a way as to cause a corresponding variation of the impedance and a consequent variation of the current flowing through the lamp;
a current sensor resistor connected in series with the lamp and connected to the said control circuit means;
the said control circuit means including signal treatment circuit means connected to the terminals of the said resistor and operable to provide at its output a square wave signal at the same frequency as the signal developed across the terminals of this resistor.
A self-oscillating circuit for driving a gas discharge lamp of the type specified above is described in earlier Italian Patent Application No. T091A000549. In this circuit the signal treatment circuit means are arranged to provide first and second square wave signals, respectively in phase and in phase opposition with the signal developed across the terminals of the current sensor resistor. These square wave signals are utilised to control the charging and discharging of two capacitors to generate first and second ramp signals in phase with the first and second square wave signal respectively. The ramp signals thus generated are compared with a reference level which varies in dependence on an external control signal. On the basis of the result of this comparison pilot signals are generated for the electronic switches of the two circuit branches of the inverter.
In the above described self-oscillating circuit disadvantages can arise if the two capacitors utilised to generate the ramp signals have different capacity values.